Human beings have long used woody biomass such as trunks, roots, leaves, and branches in various industrial fields, including paper-making, construction, feedstuff production, fuel production, and the like. Industries in which woody biomass is utilized are realized again in view of the improvement of global environmental issues because they enable the use of sustainably-usable resources in the future. Thus, such industries are expected to be recycling-oriented industries, which utilize carbon sources instead of current fossil resources. Hence, the afforestation industries mainly for fast-growing trees of the genus Eucalyptus, the genus Acacia, and the like are promoted throughout the world in order to stably and sustainably secure woody biomass.
Woody biomass is composed of vessel cells and wood fiber cells existing in the secondary xylem of plant stems. These cells are both characterized by the formation of secondary cell walls within the cells, which are composed of cellulose, hemicellulose and lignin. Wood fiber cells which account for most of the secondary xylem are used as woody biomass in industrial fields (Non-patent Document 1). The amount of secondary cell walls (the degree of thickness) of the wood fiber cells is very important because it influences the amount of biomass or the physical properties of wood fibers.
In recent years, bioenergy production and biofinery using woody biomass have become increasingly popular, as is well known. It is very beneficial to thicken the secondary cell walls to increase the amount of biomass in view of improvement of the productivity and cost reduction. Also, when woody biomass is regarded as a raw material for conventional paper-making, it is very beneficial to thicken the secondary cell walls to increase the amount of biomass in view of improvement of the productivity and cost reduction for bulky paper and the like, for which wood fibers having thick secondary cell walls and high Runkel ratio (secondary cell wall-to-lumen ratio) levels are required.
In concert with the future development of afforestation program, the formation of secondary cell walls of wood fiber cells as major source of woody biomass is promoted to change the amount of biomass (cellulose, hemicellulose, and lignin) and to change the morphology of wood fibers (e.g., the Runkel ratio). Thereby, future use for energy or expanded applications as industrial raw materials can be expected. Therefore, the development of a method for promoting the formation of secondary cell walls of wood fiber cells is very important for realizing more effective and efficient production of woody biomass on a global scale.
Several methods for promoting the secondary cell wall formation have become known to date (Patent Documents 1 to 5 and Non-patent Documents 2 to 5). However, they cannot be put to practical use because many of them relate to the promotion of the formation of secondary cell walls of plant cells other than wood fibers, and the amounts of cells and secondary cell walls resulting from the promotion are extremely low. Further, although there are techniques for promoting the formation of secondary cell walls of wood fiber cells, they cannot be practically used because adverse effects such as dwarfing are observed. As described above, it is difficult to promote the formation of secondary cell walls of wood fiber cells using the currently known techniques.
The present inventors have found many genes whose expression is specifically induced upon the formation of vessel cells (Non-patent Document 6). The group of genes found by the present inventors comprises many genes of unknown functions, and these genes are considered to play some roles in vessel formation. Vessel cells and wood fiber cells are highly analogous to each other in that both cells form secondary cell walls inside the cells, and they both are said to originate from tracheids. Therefore, it is considered that the analysis of the functions of these genes can enable the control of the secondary cell wall formation, not only for vessel cells, but also for wood fiber cells. However, there has been no such technique or finding.